This article reviews the effectiveness of 33 early childhood stimulation interventions in children 0-3 year-old in developing countries. The article explores the conditions under which these programs work, and for what populations they have greater effectiveness. We conclude that the interventions of greater intensity and longer duration are the most effective. Moreover, for these interventions to be effective they must focus on young and disadvantaged children, and actively seek the participation of families and caregivers. However, more studies are still needed to determine the age and type of optimal delivery mode.